PSS: Periodic Steady State
It's used to find Steady State response of almost all kind of circuits.Once Steady State response is found, Frequency Response,Spectral Input/Output,Phase,Time Domain response etc. is found.
It's generally used for nonlinear excited or autonomous ( oscillators ) circuits.The simulator tries to find Steady State position of the circuit by either Newton Shooting Method or Harmonic Balance, once is circuit seems "stable" meaning all voltages and currents are almost as same as in previous period, then the simulator calculates ( FFT, Frequency,Phase,Voltages,Currents,Powers, etc. ) all kind of responses mentioned above.
More information, see relevant Cadence help files.